netdev - Re: Stable regression with 'tcp: allow splice() to build full TSO packets'

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Date:	Fri, 18 May 2012 00:10:58 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Willy Tarreau <w@....eu>
Cc:	netdev@...r.kernel.org
Subject: Re: Stable regression with 'tcp: allow splice() to build full TSO
 packets'

On Fri, 2012-05-18 at 00:02 +0200, Eric Dumazet wrote:
> On Thu, 2012-05-17 at 23:50 +0200, Eric Dumazet wrote:
> > On Thu, 2012-05-17 at 23:40 +0200, Eric Dumazet wrote:
> > 
> > > I dont understand why we should tcp_push() if we sent 0 bytes in this
> > > splice() call.
> > > 
> > > The push() should have be done already by prior splice() call, dont you
> > > think ?
> > > 
> > > out:
> > > 	if (copied && !(flags & MSG_SENDPAGE_NOTLAST))
> > > 		tcp_push(sk, flags, mss_now, tp->nonagle);
> > > 
> > 
> > I think I now understand
> > 
> > One splice() syscall actually calls do_tcp_sendpages() several times
> > (N).
> > 
> > Problem is N-1 calls are done with MSG_SENDPAGE_NOTLAST set
> > 
> > And last one with MSG_SENDPAGE_NOTLAST unset
> > 
> > So maybe we should replace this test by :
> > 
> > 	if (!(flags & MSG_SENDPAGE_NOTLAST))
> > 		tcp_push(...);
> > 
> > 
> 
> Its more tricky than that.
> 
> If we return 0 from do_tcp_sendpages(), __splice_from_pipe() wont call
> us again, but we need to flush().  (This bug is indeed very old)
> 
> So maybe use :
> 
> if ((copied && !(flags & MSG_SENDPAGE_NOTLAST)) ||
>     !copied)
> 	tcp_push(...);


But then your patch is equivalentm so I'm going to Ack it ;)


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